1. Field of the Invention
The present invention relates to a transmitter and a receiver, which are used for an MC-CDMA system based on a multi-carrier CDMA (hereinafter referred to as MC-CDMA) method.
2. Description of the Related Art
In a mobile radio communication, attention is focused, as an ultra-high speed radio access technology in a frequency selectivity fading environment, to the MC-CDMA system which results from the merging of the OFDM method and the CDMA method. This system is a system in which to transmit a transmitting signal, it is multiplied by the code sequence, proper to each user, in a frequency domain to be spread and split into a plurality of subcarriers (see “Application of OFDM Modulation Method”, written by Wataru Matsumoto and Hideki Ochiai, Triceps Corporation, 2001).
A conventional configuration containing a transmitter and a receiver in this system is shown in FIG. 4.
In FIG. 4, d(k) represents the data of a user k. At this time point, the data is digital data. Each QPSK modulator part receives data of 2 bits and output the data in a form of one symbol. Each copy part copies the data by a spreading ratio on the frequency axis. L represents spreading ratio. The copying operation is performed the number of times corresponding to the number of spreading codes. Cm, k represents a spreading code of the m-th subcarrier of a k-th user. The spreading code used is the Walsh spreading code shown in FIG. 1. Cm, k indicates a spreading code at the m row and the k column in FIG. 1. Each multiplexing part multiplexes spreaded signal of users. A pilot symbol insertion part inserts a pilot symbol for checking conditions of a propagation path in a demodulator side to the output signal from the multiplexing part. Generally, when transmission data is restored into its original data, even if the process in the transmission side is reversely executed, the data is not restored to the original one since the data has undergone the delay and the fading phenamenon. To cope with this, a known pilot symbol is inserted, and the receiving side estimates conditions of the propagation path on the basis of the pilot symbol received. And, the receiving side demodulates the receiving data by using the estimated information (the delay profile and the frequency response of the propagation path). An IFF (Inverse Fast Fourier Transform) part performs the IFFT to convert the frequency spectrum to a time signal. A guard interval addition part is provided for taking a measure for delay waves, and prevents the previous symbol from entering the sample when the demodulation side performs the FFT (fast Fourier transform). With provision of the guard interval, when delay occurs, the trailing part of a signal is located at the leading part of the signal. From this, it is recognized that the signal has been cyclically shifted by the delay. In the receiving side, the guard interval addition part removes the guard interval from the receiving data and executes the FFT process to converts the time signal to the frequency spectrum. The receiver extracts the pilot symbol for estimation from the spectrum, and a propagation path estimation part estimates a frequency response hm of the propagation path on the basis of the extracted pilot symbol and a reception pilot symbol. Then, a weight factor computing part weights a spreading code Cm, k and frequency response hm, to obtain the k-th user and a weighting factor Gk, m of the m-th subcarrier, and to convert one symbol that is input in the QPSK modulator part into 2-bit data.
In the MC-CDMA system thus constructed, the Walsh code has been used for the spreading code. Where such a spreading signal is used, however, when it receives the influence of the delay wave, the spreading codes of the users lose their orthogonality. Its influence affects plural users, and the error rate characteristic is deteriorated. To cope with this, there are proposed an orthogonality restoring combining (ORC) method which uses for composition the weight factors at the time of demodulation, and a minimum mean square error combining (MMSEC) method. And it is confirmed that those proposals are effective for the measure  (see, N. Yee, and J P. Linnartz,” Controlled Equalization of Multicarrier CDMA In Indoor Rician Fading Channel”, “Proc. IEEE VTC” 94, pp. 1665-1669, 1994; A. Chouly, A. Brajal, and S. Jourdan, “Orthogonal multicarrier techniques applied to direct sequence spread spectrum CDMA systems,” Proc. IEEE GLOBECOM, '93, pp 1723-1728, September 1999; and S. Hara and R. Prasad, “Design and Performance of Multicarrier CDMA systems in Frequency-Selective Rayleigh Fading Channels”, “IEEE Trans. Veh. Technol., Vol. 48, pp 1584-1595, September 1999).
Also in such methods, however, in case where the multiplex number increases, it is impossible to completely keep orthogonality, and the BER characteristic is deteriorated by the inter-channel interference.